Optical bandpass filers transmit light over a pre-determined band of wavelengths while rejecting, by absorption, radiation or scattering, all other wavelengths. Such filters are useful in laser cavities or optical communications systems. For example, they may be used to constrain the wavelength of operation of a laser, when deployed inside or outside the laser cavity. In optical communications systems, they can be used at the input of an optical receiver to separate unwanted light such as spontaneous emission noise outside the wavelength band of the signal. See D. M. Shamoon, J. M. H. Elmirghani, R. A. Cryan, “Characterisation of optically preamplified receivers with fibre Bragg grating optical fibers”, IEEE Colloquium on Optical Fiber Gratings, March 1996. Optical regenerators based on self-phase modulation require extracting a predetermined wavelength band from a broad spectrum of light.
Several devices have been proposed and demonstrated to offer the functionality of bandpass filtering. Fiber Bragg gratings may be used in the reflection mode with a circulator, or in the transmission mode, to select a narrow wavelength band. See Xu, M. G.; Alavie, A. T. ; Maaskant, R.; Ohn, M. M.; “Tunable fiber bandpass filter based on a linearly chirped fiber Bragg grating for wavelength demultiplexing”, Electron Lett., 32, pp. 1918-1919 (1996). Operation in the reflection mode requires addition of a circulator in the transmission line—this increases cost, loss and complexity for the device. A further drawback with Bragg gratings, used either in transmission or reflection, is that such filters can be highly dispersive, which gives rise to pulse shape distortions. See Lenz, G.; Eggleton, B. J.; Giles, C. R.; Madsen, C. K.; Slusher, R. E.; “Dispersive properties of optical filters for WDM systems”, IEEE Journal of Quantum Electronics, 34, pp. 1390-1402, (1998). Alternatively, thin film dielectric filters may also be used as bandpass filters (see: U.S. Pat. No. 5,615,289), but in addition to their dispersive nature (similar to Bragg gratings) they suffer from the additional drawbacks of being free-space devices. Thus, light needs to be coupled into fiber pigtails for use in such fiber-optic systems. This increases loss, cost and complexity.
An alternative technique for making bandpass filters uses two identical long-period fiber gratings (LPGs) that are spliced in series in the fiber-optic transmission line, with a core block between them. (See: U.S. Pat. No. 6,151,427.) The first long period grating converts a narrow wavelength-band of core-mode light into a cladding mode, and the second identical grating couples the cladding-mode light back into the core mode. The core block between the two LPGs attenuates or scatters any light that was not converted into the cladding mode. There are drawbacks associated with this device—(1) the core block simultaneously attenuates light in all the modes, thus the device is inherently lossy. Consequently only higher-order cladding modes may be utilized, as lower order cladding modes will exhibit even higher loss; (2) the core block forms a discrete discontinuity in the fiber, which leads to undesired mode-coupling and inter-modal interference; (3) tuning such a filter requires simultaneously tuning both LPGs by identical amounts, as the filter operates properly only when both LPGs have identical spectra. In bandpass filters using acousto-optically generated or microbend-induced LPGs, there is an additional drawback: the device is inherently polarization sensitive.
Thus, there exists a need for a bandpass filter that is an in-line fiber device, has low loss, is polarization insensitive, tunable, and simple to implement.